Abstract
In his post-1720 works, Berkeley focuses his comments about Descartes on mechanism and about Locke on general abstract ideas. He warns against using metaphysical principles to explain observed regularities, and he extends his account to include spiritual substances (including God). Indeed, by calling a substance a spirit, he emphasizes how a person is simply the will that ideas be differentiated and associated in a certain way, not some <i>thing</i> that engages in differentiation. In this sense, a substance cannot be conceived apart from its activity.